Various liquid dispensing systems have been developed to dispense a liquid spray pattern from a nozzle outlet toward a surface of a substrate. For example, in conformal coating applications, liquid dispensing systems have been designed to dispense flat, fan-like liquid spray patterns of conformal coating material onto a surface of the substrate, such as a printed circuit board. The dispensing nozzle of these systems may be a cross-cut, slit or air-assisted slot nozzle, for example, that is configured to dispense the coating material as a spray, continuous band or sheet, or fibrous web of a predetermined pattern width toward the circuit board. The dispensing nozzle is typically moved by a robotic movement platform in opposite back-and-forth directions relative to the circuit board to dispense side-by-side tracks or bands of conformal coating onto the circuit board and thereby obtain a uniform moisture resistant barrier on the board surface. Alternatively, the circuit board may be moved relative to the liquid dispenser which may be fixed.
During the conformal coating process, it is important that the tracks or bands of coating join or converge along their adjacent edges, or even slightly overlap, to ensure that a full surface coating is provided on the board. Otherwise, the circuit board will be left vulnerable to undesirable chemical or moisture attack by any gaps left between the coating tracks or bands. On the other hand, too much overlap of the adjacent edges may cause undesirable bubbles in the thickened coating areas at the overlap that may also jeopardize quality control. Thus, a consistent width of the liquid spray pattern is generally necessary to obtain a uniform layer of conformal coating on circuit boards.
Unfortunately, while a liquid material dispenser may adequately and reliably dispense uniform conformal coating layers on circuit boards in one production run, changes in the viscosity and/or fluid pressure of the material will often lead to undesirable variations in the liquid spray pattern width. Additionally, contamination or partial blockage of the nozzle outlet will cause undesirable changes in the spray pattern. If these changes are not detected prior to a production run, improperly coated boards must be reworked and costly downtime of the conformal coating system is typically required to identify the problem and manually adjust the liquid dispensing system to obtain the desired pattern width and minimal offset. As those skilled in the art will readily appreciate, pattern width control is also critical in other liquid dispensing applications as well, such as in paint, flux and adhesive dispensing environments. In each of these applications, very often the edge positions of the dispensed liquid pattern relative to a substrate surface or to an adjacent liquid pattern dispensed on the substrate must be properly adjusted and set to obtain the desired material application.
Thus, there is a need for a control system for use in a liquid dispensing system that verifies the width of the dispensed liquid spray pattern.
There is also a need for a control system for use in a liquid dispensing system that improves the operator's ability to readily identify problems in the dispensed liquid spray pattern.